Power control system

ABSTRACT

In power control including at least two thyristors disposed between a dc input and an ac output and at least one capacitor interconnecting the thyristors in pair, a nonlinear reactor is serially connected to each thyristor and one of the thyristors is fired to apply a voltage accumulated on the capacitor, to the other thyristor, as a reverse bias, to effect commutation. A diode can be connected to reverse parallel to each thyristor. Also a commutation transformer can be disposed in a path along which the voltage across the capacitor is applied to either of the thyristors.

O United States Patent 1151 3,683,267 Akamatsu 45 A 8, 1972 [54] POWERCONTROL SYSTEM 3,349,315 10/1967 Studtmann ..32l/45 Inventor: M ikAkamatsu, g Katz 8t 3] l Japan 3,405,346 10/1968 Krauthamen. ..321/453,417,315 12/1968 Corey ..321/45 Assigheer Mitsubishi Denki KlhushikiKeisha, 3,422,342 1/1969 Jackson ..321/45 Tokyo, Japan 3,423,665 1/1969Greenberg et al. ..321/45 x [22] Filed; 1 9 3,465,233 9/1969 Johnston etal ..32l/45 [21] App]. No.: 881,225 Primary Examiner-William M. Sh00p,Jr.

Attorney-Robert E. Burns and Emmanuel J. Lobato [30] ForeignApplicationfriority Data [57] ABSTRACT Dec. 14, 1968 Japan ..43/9l8l4Dec. 26, 1968 Japan ..43/957s1 In power control including at least twothyristors Feb. 3, 1969 Japan ..44/s033 disposed between a dc input andan 89 Output and at Dec. 2, 1968 Japan ..'43/88144 least one capacitorinterconnecting the thyristors in F b 3, 1969 Japan 44/8()34 pair, anonlinear reactor is serially connected to each Feb. 19, 1969 Ja an44/12394 thyristor and one of the thyristors is fired to apply a July 2,1969' Japan ..44/57571 v ltag a um n the capacitor, t0 the otherthyristor, as a reverse bias, to effect commutation. A [52] US. Cl...,.....321/45 R, 321/45 C diode can be connected to reverse parallel toeach [51] Int. Cl. ..H02m 7/48 thyristor. Also a commutation transformercan be [58] Field of Search ..32l/43-45, 45 C disposed in a path alongwhich the voltage across the capacitor is applied to either of thethyristors. [56] References Cited 13 Claims, 30 Drawing Figures UNITEDSTATES PATENTS 3,340,457 9/1967 Schmitz ..321/45 PKTE'N'TEDAuc a maSHEET 10F 7 V IPR/ORART P'A'IENTEDM 1912 31383.26?

SHEET 2 BF 7 PATENTEUma 8 I972 3.683, 257

sum 5 or 7 FIG. 18'

FIG. 21

PAIENTEM: 81912 SHEET 7 0F 7 FIG. 25

FIG. 26

FIG. 27

FIG 26 POWER CONTROL SYSTEM BACKGROUND OF THE INVENTION This inventionrelates to a power control system having a direct current power appliedto the input thereof and including thyristors for controlling analternating current output therefrom or converting the direct currentpower to an alternating current power.

As power control systems of the type referred to there have beenpreviously widely imployed McMurrey and Bed-Ford type inverters. Thattype of inverters has been operated to have a period of time for whichan energy accumulated on a commutation reactor involved is released andwhich could amount to about to 50 times an interval of time for whichthe associated thyristor is reversely biassed. Within the release periodof time a current flowing through the thyristor gradually decreases fromabout two time a peak magnitude of maximum output current inapproximately rectilinear or exponential manner.

What particularly comes into question is a long transient commutationtime within which a ratio of a commutation current to a load current ishigh and particularly when the operating frequency is high. Thereforethe thyristors have decreased in current utilization and the actuallyoperating .frequency is restricted in upper limit. Further the releaseof the energy accumulated on the commutation reactor as above describedhas lead to a loss of energy because such energy is delivered to eitherthe source of electric power through a transformer or to the associatedresistor. In addition, the commutation reactor has included a magnetcore having an air gap leading to the generation of the noise.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention toprovide a new and improved power control system low in commutationcurrent, short in transient commutation time, and decreased in magnitudeof a current required for each of thyristors involved as well asincreasing the upper limit of the practically operating frequency.

It is another object of the invention to provide a new and improvedpower control system decreased in loss of commutation thereby toincrease the system efficiency.

It is still another object of the invention to provide a new andimproved power control system minimized in noise level.

It is a further object of the invention to provide anew and improvedpower control system small-xized and in expensive to be manufactured.

It is an additional object of the invention to provide a power controlsystem including an improved commutation reactor capable of easilyresetting a magnetic flux therein to its unsaturated region.

It is still another object of the invention to provide a new andimproved power control system in which a voltage applied across athyristor involved is suppressed from increasing and a magnetic flux incommutation reactor is rapidly reset to its unsaturated region.

The invention accomplishes the above cited objects by the provision of apower control system comprising a pair of terminals for a source ofdirect current, a pair of alternating current output terminals, at leasttwo semiconductor controlled rectifiers each connected one of the directcurrentterminal and one of the alternating current output terminals, acommutation circuit including the semiconductor controlled rectifiers toalternately switch the latter in a predetermined order, at least acommutation capacitor connected in the commutation circuit and a currentpath for supplying a load current through the semiconductor controlledrectifiers, characterized in that one reactor is connected in seriescircuitrelationship to each of the controlledrectifiers in the currentpath.

The reactor maybe preferably a nonlinear reactor.

Advantageously, the commutation circuit may include, in addition to thecapacitor, at least a reactor element and one semiconductor diode may beconnected in reverse parallel circuit relationship to each of thesemiconductor controlled rectifiers. Alternatively the commutationcircuit may be formed of at least the commutation capacitor and acommutation transformer.

In order to prevent a voltage applied across each of the semiconductorcontrolled rectifiers from increasing above a voltage across the directcurrent terminals, one clamping semiconductor diode may be connectedbetween the junction of the first-mentioned reactor and the associatedsemiconductor controlled rectifier and one of the direct currentterminals.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readilyapparent from the following detailed description taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a graphic representation of commutation waveforms developed inthe well known McMurrey and Bed-Ford type of inverters;

FIG. 2 is a schematic circuit diagram of a power control systemconstructed to provide an improvement over systems such as describedwith respect to FIG. 1;

FIG. 3 is a graphic representation of commutation waveforms developed onthe systemillustrated in FIG.

FIG. 4a is a schematic circuit diagram of a power control systemconstructed in accordance with the principles of the invention;

FIG. 4b is a graphic representation of commutation waveforms developedin the system illustrated in FIG.

FIG. 5 is a schematic circuit diagram of a bridge type power controlsystem constructed in accordance with the principles of the invention;

FIG. 6 is a schematic circuit diagram of a modification of the systemillustrated in FIG. 5;

FIG. 7 is a schematic circuit diagram of another modification of thesystem illustrated in FIG. 4a;

FIG. 8 is a schematic circuit diagram of a modification of theinvention;

FIG. 9 is a schematic circuit diagram of a modification of the systemillustrated in FIG. 8;

FIGS. 10 and 11 are schematic circuit diagrams of differentmodifications of the system illustrated in FIG.

FIG. 12 is a schematic circuit diagram of another modification of theinvention;

FIGS. 13 through 15 are schematic circuit diagrams of differentmodifications of the system illustrated in FIG. 12;

FIG. 16 is a graphic representation of commutation waveforms developedon the system illustrated in FIG.

FIGS. 17 through 19 are schematic circuit diagrams of variousmodifications of the system illustrated in FIG. 15;

FIG. 20 is a schematic circuit diagram of still another modification ofsystem illustrated in FIG. 12;

FIGS. 21 and 22 are schematic circuit diagrams of differentmodifications of the system illustrated in FIG. 20;

FIG. 23 is a schematic circuit diagram of another modification of thesystem shown in FIG. 12;

FIG. 24 is a schematic circuit diagram of another modification of thesystem illustrated in FIG. 15 and illustrating a structure of acommutation transformer involved;

FIG. 25 is a perspective crosssectional view of a modification of thetransformer illustrated in FIG. 24;

FIG. 26 is a graph illustrating a magnetic hysteresis loop for amagnetic core used in a nonlinear reactor constructed in accordance withthe principles of the invention;

FIGS. 27 and 28 are diagrams useful in explaining the manner in which amagnetic core of a nonlinear reactor involved is reset to an unsaturatedflux region according to the principles of the invention; and

FIG. 29 is a view similar to FIG. 26.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand FIG. 1 in particular there is illustrated commutation waveformsdeveloped on the McMurrey and Bed-Ford type of inverters as previouslyoutlined. In FIG. 1 waveforms A and B represents a voltage appliedacross and a current flowing through thyristors involved respectively,at the beginning of a transient commutation time r a thyristor is firedto be transiently applied with a high voltage V as shown in FIG. 1A.During the commutation time a voltage less than the voltage V but stillhigher than a voltage across the associated source of direct current(not shown) continues to be applied across the thyristor and after thecommutation time the thyristor has applied thereacross the voltageacross the source. It is noted that the commutation time t is long asshown in FIG. 1.

Also FIG. 1 shows at current waveform B that, upon turning on thethyristor, a commutation current flowing therethrough abruptly increasesto that magnitude I,, of a load current flowing therethrough just beforethe particular commutation has been effected leading to a great increasein time differential coefficient of the current or di/dt. This resultedin critical conditions for operating the system and the higher thecurrent and voltage for or the capability of the system that di/dt wouldcome into question.

Also due to the long commutation time r a ratio of an integratedcommutation current i (which is shown by a hatched portion) to anintegrated load current 1), has increased and particularly when theoperating frequency is high. For example, with an inverter formed ofthyristors having a turn-off time of 30 microsecond to be operated at afrequency in the order of 100 Hz the proportion of the commutationcurrent contributing to an increase in temperature of the thyristor hasamounted to about 30 percent and it has often exceeded 50 percent.

Thus the thyristors have decreased in current utilization leading to adecrease in output available with the same thyristor or to an increasein current capacity required for thyristors used. This resulted in therestriction of the upper limit of the practically operating frequency aspreviously described.

As previously described, an energy accumulated on the commutationreactor has been at least partly lost. If the energy has been fed backto the associated source through a transformer, the feedback efiiciencyhas been very low due to the leakage inductance and resistance of thetransformer and most of the energy has been lost. Alternatively, if theenergy is delivered to the associated resistor, all the energy has beennot only lost but also a loss due to that portion of the load currentflowing through the resistor has been added to the former loss,resulting in a great loss of commutation and therefore in a decrease inefficiency. Thus an increase in current required for the thyristor and adecrease in efficiency has particularly come into question for highcapability and relatively high frequency apparatus.

In addition, the use of a magnetic core of a commutation reactor havingan air gap has caused the noise as previously described.

In order to eliminate the above-mentioned disadvantages, the circuitryas shown in FIG. 2 has been proposed. The arrangement illustrated inFIG. 2 comprises a source of directcurrent E having a pair of directcurrent (dc) terminals. One of the terminals, in this example thepositive terminal P is connected to a center tap on a primary winding Tof an output transformer T and the other or negative terminal N isconnected to either end of the primary winding T through a seriescombination of nonlinear reactor and thyristor 1a and 2a or lb and 2b.The junction of the nonlinear reactor la and the thyristor 2a isconnected to the junction of the reactor 1b and the thyristor 2b througha commutation capacitor 3 for serving to effect commutation between thethyristors 2a and b, a main semiconductor diode 4a or b is connectedacross the associated series combination 1a2a or 1b-2b and poledoppositely to the thyristor 2a or 2b respectively. The outputtransformer T includes a secondary winding T having a pair of outputterminals 0 and 0 across which is connected a load Z The nonlinearreactor la, thyristor 2a and diode 4a are substantially identical inconstruction and operation to the corresponding components lb, 2b and 4brespectively.

The operation of the arrangement as above described will now bedescribed. Assuming that one of the thyristors, for example thethyristor 2a is in its conducting state by applying to its gateelectrode a gating voltage from any suitable source of voltage (notshown). The output transformer T have developed across the primarywinding T a voltage having a polarity illustrated beside the winding andthe capacitor 3 has been charged with voltage substantially equal to thevoltage across the terminals P and N and having a polarity illustratedbeside the same while the nonlinear reactor 2a is maintained saturated.

The succeeding firing of the other thyristor 2b permits the voltageacross the capacitor 3 to reversely a bias the conducting thyristor 2ato render it nonconductive. Within the nonconducting period of time forthe thyristor 2a, a current flows from the end labelled the symbol ofthe primary transformer winding T through the nonlinear reactor 1b andthe now conducting thyristor 2b but it is suppressed by an unsaturatedimpedance presented by the nonlinear reactor 1b.

On the other hand, the nonlinear reactor la has supplied thereto avoltage equal to the reverse voltage applied across the thyristor 2athrough the diode 4a. Thus an oscillation current flows through a closedloop including the thyristor 2b, the diode 4a, the reactor lb and thecapacitor 3 whose frequency is determined by the saturated residualinductance of the reactor 1a and the capacitance of the capacitor 3 withthe result that the nonlinear reactor 1a is reset to its unsaturatedflux region. This causes the capacitor 3 to charge with the sum of aload current having flowed through the thyristor 2a just before thelatter became nonconducting and the said oscillation currentprogressively increased in amplitude due to the saturated inductance ofthe reactor 1a but with the polarity reverse from that during theconduction of the thyristor 2a.

In other words, if the system is unloaded at that time then thecapacitor 3 is rapidly charged with the oscillation current alone.Alternatively if the system is loaded then the capacitor is more rapidlycharged with the resultant magnitude of the particular load current andthe oscillation current. The more the loading on the system the higherthe charging rate of the capacitor will be.

On the other hand, the now conducting thyristor 2b has flowingtherethrough a current reversely charging the capacitor 3 whilea currentfrom the reactor 1b is of a minimum magnitude due to the unsaturatedinductance of the reactor.

After the completion of the particular commutation, a returning currentfrom the load Z flows through the diode 4b while what flows through thethyristor 2b is formed of an exciting current for the nonlinear reactor2b very low in magnitude and decayed.

After the completion of the particular commutation the load current isreversed in polarity whereupon the nonlinear reactor 1b is saturated tocause the load current to flow through the conducting thyristor 2b. Thenthe conducting thyristor la changes from the thyristor 2b to thethyristor 2a to repeat the process as above described.

Thus it will be appreciated that by using the nonlinear reactors, anenergy previously accumulated on one of the nonlinear reactors israpidly transferred to the capacitor, during the commutation time periodwhile an energy accumulated in the other reactor is very low whereby thecommunication current is allowd to flow through the particular thyristoronly for a short time resulting in the communication current averaging avery small magnitude.

FIG. 3 wherein like reference characters have the same meaning as in FIG. 1 illustrate waveforms of voltage and current developed in thesystem of FIG. 2. By comparing FIG. 1 with FIG. 3 it will be seen thatin the arrangement of FIG. 2 the commutation time t becomes equal tofrom three to six times a time for which either of the thyristor isreversely biased. That is, as compared with the systems described inconjunction with FIG. 1 the commutation time of the circuitry of FIG. 2reduces by a factor of about seven and the mean commutation currentreduces by a factor of from about 5 to 10.

In the arrangement of FIG. 2 the re-charge rate for the commutationcapacitor 3 greatly depends upon the magnitude of the load current. Thisleads to a long commutation time tc under light loading. Therefore, adrecrease in commutation time has been required to decrease theinductances of the reactors 1a and b. Alternately, it has been necessaryto rapidly saturate the reactors.

However, a decrease in inductance of each reactor la or b has caused anincrease in current flowing through the capacitor 3, the thyristor 2b,the diode 4a and the reactor 1a during the commutation, thus leading tothe necessity of using a large commutation capacitor. Simultaneously,the reactor 1b has drawn a higher current from the source of directcurrent Ed resulting in a further increase in capacitance of thecommutation capacitor.

In addition, the energy accumulated in the reactor lb during thecommutation time increases and causes a corresponding increase in energyaccumulated in the commutation capacitor. Thus, the commutation loss isincreased. The increase in energy accumulated by the reactor lb isaccompanied by an overshoot voltage or an oscillating voltage componentacross the thyristor as shown at V in FIG. 3.

The invention contemplates the elimination of the disadvantages justdescribed and provides a power control system of shortened commutationtime, low in overshoot voltage or peak voltage across the thyristorconnected in the system, and low in commutation loss.

Referring now to FIG. 4a there is illustrated a power control systemconstructed in accordance with the principles of the invention. Thearrangement illustrated is similar to that shown in FIG. 2 but alsoincludes semiconductor diodes 6a and 6b connected respectively inreverse parallel circuit relationship across each of the thyristors 2aand b, as well as a commutation reactor 7 serially connected to thecommutation capacitor 3. Therefore, like reference characters have beenemployed to identify the components identical to those shown in FIG. 2.

In operation the commutation reactor 7 cooperates with the commutationcapacitor 3 to permit an oscillating pulse current to flow therethroughupon the commutation, whereby the commutation from one to the other ofthe thyristors is rapidly completed. For example, upon effecting thecommutation from the thyristor 2a to the thyristor 2b, the conduction ofthe thyristor 2b causes the electric energy charged on the commutationcapacitor 3 to oscillate in a closed loop traced from the commutationcapacitor 3 through the thyristor 2b, the diode 6a and the commutationreactor 7, and thence to the capacitor 3. Therefore, the thyristor 2a isreversely biased only when the resulting oscillating pulse current is inexcess of a current flowing through the nonlinear reactor 1a. Then, at atime point when the thyristor 2a is reversely biased through theabove-described oscillation, that is to say, when the diode 6a is fullyconducting, the capacitor 3 is already charged with the polarityreversed from that illustrated.

It has been found that the oscillation operation as above-described isrequired only to have its half period equal to about twice or threetimes the turn-off time of the thyristor 2a. Therefore, in thearrangement of FIG. 4a the time interval for which the commutationcapacitor 3 is re-charged, or the commutation time tc, is extremelydecreased'to permit the high speed switching operation. In addition, thecommutation time depends upon the period of natural oscillationdetermined by the commutation reactor and capacitors 7 and 3,respectively, and is scarcely affected by the magnitude of the loadcurrent.

Furthermore, the nonlinear reactors la and b are serially connected tothe thyristors 2a and b, respectively, such that they are notoperatively coupled to each other. That is, the reactors are operativeindependent of each other and are therefore able to be saturated by aflow of load current therethrough. In other words, the reactors 1a and bcan be saturable reactors.

With saturable reactors used as the reactors la and b, it is possible tominimize the saturated inductance of the reactor la while rendering theunsaturated inductance to the reactor lb as high as possible. Thismeasure permits a decrease in the energy accumulated in each reactor laor b. As the overshoot voltage V, across the thyristor is affected bythe energy accumulated in the associated reactor, the same can decreaseas shown in FIG. 4b wherein like reference characters have the samemeaning as those shown in FIG. 3. Accordingly, the commutation loss canbe minimized.

FIG. 5 shows a modification of the invention embodied into a bridge typecircuit. In FIG. 5 and the succeeding Figures like reference charactersdesignate the components similar or identical to those illustrated inFIGS. 2 and 4a. As shown, the bridge circuit has four arms eachincluding a series combination of nonlinear reactor 1 and thecommutation capacitor 3 connected across the respective main diode 4with the thyristors similarly pole with respect to the source E Thejunctions of the thyristors 2a, and b, c and d are connected to thesource terminals P and N respectively and those of the nonlinearreactors la and c, b and d have the respective output terminals 0, and 0across which is connected the load Z The reactors 1a and d are shown asbeing magnetically coupled to the reactors la and b respectively.Further the junction of the reactor and thyristor la and 2a or 1c and 2cis connected to the junction of the reactor and thyristor lb and 2b or1d and 2d through the commutation capacitor 30 or b respectively.

The thyristors 2a and d are adapted to be simultaneously turned ON andOFF while the thyristors 2b and c are adapted to be simultaneouslyturned ON and OFF but alternately with respect to the thyristors 2a andd respectively. Therefore it will be appreciated that the system shownin FIG. 5 is identical in commutation mode of operation to the system ofFIG. 4a.

FIG. 6 shows an arrangement similar to FIG. 5 except for a seriallysemiconductor diode being connected between the associated nonlinearreactor and thyristor with the commutation capacitor connected betweenthe junctions of the diodes and thyristors. For example, the diode 5a isconnected between the nonlinear reactor la and the junction of thethyristor 2a and the capacitor 3 or 3a.

The diode 5 serves to prevent the associated capacitor charged to avoltage above the source voltage through the associated nonlinearreactor after the particular commutation from discharging in thedirection opposite to the charging direction.

This measure ensures that a voltage charged on the capacitor ismaintained high resulting in an increase in commutation performance andtherefore is particularly effective for using with low voltage, highcurrent apparatus.

FIG. 7 shows a modification of the system as illustrated in FIG. 4a, anddiffers in the connection of a pair of semiconductor diodes 5a and b inseries, respectively, with the reactors la and b and the thyristors 2aand b, with the same polarity as the latter. Therefore, the operation ofthe arrangement as shown in FIG. 7 will be readily understood from thedescription related to FIGS. 4a and 6.

The voltage across the capacitor 3 then increases with the polarityopposite to that illustrated to decrease the current flowing thecapacitor until that current equals the load current. At that time thecommutation diode 6a becomes nonconducting to permit a forward voltageto be applied across the thyristor 2a with the result that the capacitor3 continues to be charged in oscillatory manner through the nonlinearand commutation reactors la and 7 respectively to compensate for adeficent amount of charge thereon.

When the capacitor 3 has reached a predetermined voltage approximatelyequal to twice the source voltage, the diode 4b is fired. At that timethe nonlinear reactor 1b still remains unsaturated and the thyristor 2bhas a very low exciting current flowing therethrough while decaying.

As in the previous examples the load current is reversed in polarity tosaturate the nonlinear reactor 1b with the result that the load currentwith the reversed polarity is supplied to the load Z through thethyristor 2b. Then the thyristor 2a is again fired to repeat the processas above described.

FIG. 8 illustrates another form of the invention corresponding to thesystem of FIG. 7 with the output transformer T omitted. The nonlinearreactors 1a and b each include a pair of windings 101 and la2 or lbl andlb2 preferably inductively coupled to each other as shown in FIG. 8. Thewindings la2 and lbl are connected at one end together to the junctionof the diodes 4a and b and to the output terminal 0. The dot conventionis used to identify the instantaneous polarity of the each winding. Thewinding la2 is serially connected to a cathode electrode of asemiconductor diode 5c having an anode electrode connected to the anodeand cathode electrodes respectively of the commutation diode 6a andthyristor 2a. Similarly the winding 1b2 is connected to a semiconductor5d and thence to the commutation diode 6b and the thyristor 2b.

The cathode electrodes of both the thyristors 2a and b areinterconnected through a series combination of reactor 7b and capacitor3b while the anode electrodes thereof are interconnected through aseries combination of reactor 1a and capacitor 3a. In other respects thearrangement is substantially similar to that shown in FIG: 7.

The arrangement as above described is operated as follows: Assuming thatthe thyristor 2a is in its conducting state, a load current i flows fromthe terminal P of the dc source E through one winding lal of thenonlinear reactor la, the series diode a, the now conducting thyristor2a, the series diode 5c, the other winding 1a2 of the reactor 1a, theoutput terminal 0 and the load (not shown) and back to the terminal N ofthe source. Under these circumstances, the nonlinear reactor la is inits saturated state due to the flow of output current therethrough andscarcely presents an impedance to the load current.

At the same time the capacitor 3a is discharging through a circuitincluding the components On the other hand, the capacitor 3b is chargingwith a polarity illustrated beside the same through a circuit includingthe components until it is charged to the source voltage and with thepolarity illustrated beside the same.

Then upon firing the thyristor 2b the voltage across the capacitor 3b isoperated to initiate the output or load current 1",, flowing through thewindings M2 and lb] of both the nonlinear reactors 1a and b up to thattime to be shunted in oscillatory manner through a circuit including thecomponents as shown at current i in FIG. 8. Thus the current i flowingthrough the thyristor 2a decreases by a magnitude equal to that of theshunted oscillation current The shunt current i increases until it iseventually higher than the load current i At that time the thyristor 2ais deenergized and the commutation diode 6a is fired. A current iflowing through the fired diode 6a is equal to a difference between theoscillatory discharge current i from the capacitor 3a and the loadcurrent 1) In this way the capacitor 3b discharges from the sourcevoltage with the polarity illustrated to zero voltage whereas thecapacitor 3a charges from zero voltage to the source voltage with thepolarity illustrated in FIG. 8.

The oscillatory discharge current i reaches its maximum magnitude whenthe voltages across the capacitors 3a and b are equal to each other. Thecurrent i continues to decrease while the commutation reactors 7a and bfunction to effect oscillatory charge and discharge of the capacitorsuntil the oscillatory charge or discharge current i equals the loadcurrent 1),. At that time the commutation diode 6a is brought into itsnonconducting state and also the capacitor 3b has been substantiallycompleted to discharge while the capacitor 3a has been substantiallycompleted to charge. In other words, the energy accumulated on thecapacitor 3b is only transferred to the capacitor 3a by means of theaction of the oscillation effected by the capacitors 3a and b and thecommutation'reactors 7a and b with a minimum loss in energy occurringduring that transfer. The commutation time for which the energy is transferred from one to the other capacitors corresponds to one half theperiod of natural oscillation of the capacitors 3a and b and commutationreactors 7a and b and has been selected to be in the order of twice thetumoff time of the thyristor. Therefore the transfer of the energy fromone to the other capacitors is completed within a very short time.

After the lapse of a period of time for which one of the thyristors isreversely biassed, the capacitor 30 is additionally charged through acircuit including the components thereby to supply a slight amount bywhich each of the capacitor is to be additionally charged or discharged.Then the capacitor 3b has discharged to a zero voltaged and thecapacitor 3a has charged to the source voltage whereupon the main diode4b is fired. This forms a closed loop traced from the terminal P of thesource E through the component and back to the terminal N of the source.This causes the nonlinear reactor 1a to be reset to its unsaturated fluxregion.

Thereafter, the series diodes 5a and c I serve to prevent the closedloop just described from continuously effecting a damped oscillation dueto the residual inductances of the reactor windings lal and la2cooperating with the commutation reactors 7a and b and the capacitors 3aand b. Therefore the capacitor 3a is additionally charged in oscillatorymanner to maintain the voltage thereacross at a maximum peak magnitudeof the oscillatory voltage. This causes increase in commutationcapability and therefore the system of the FIG. 8 is particularlysuitable for the low voltage, high current applications. Morespecifically, the series diode 5a serves to prevent the capacitor 3afrom discharging while the series diode 5c serves to prevent thecapacitor 3b charged with the polarity opposite to that illustrated inFIG. 8 from discharging. This is true in the case of the series diodes5b and d. It is to be noted that any one or more of the series diodes5a, b, c and d may be omitted, if desired.

Then the load current i is reversed in polarity and the nonlinearreactor lb is saturated. Under these circumstance the load current flowsthrough a circuit including the load (not shown), and the componentsThen the process as above described is repeated.

FIG. 9 shows a modification of the system of FIG. 8 wherein the windingslal, 1a2, lbl and 1b2 of both nonlinear reactors la and b are disposedso as not to be inductively coupled to one another while the seriesdiodes 5a, b, c and d are omitted. In other respects the arrangement isidentical to that shown in FIG. 8. Therefore it will be understood thearrangement is quite identical in operation to that shown in FIG. 8, andthe description need not be further made.

FIG. 10 illustrates an arrangement substantially identical to that shownin FIG. 4 excepting that a series combination of commutation reactor andcapacitor substitutes the commutation capacitor shown in FIG. 4 and thatthe thyristors have respective commutation semiconductor diode connectedin reverse parallel relationship thereto. For example, a seriescombination of commutation reactor and capacitor 7a and 3a substitutesthe capacitor 3a shown in FIG. 4 and the commutation diode 6a isconnected across the thyristor 2a. Also the thyristors 3a, b, c and dare identical in operation to those shown in FIG. 4. Therefore thearrangement need not be further described.

A modification of the system shown in FIG. 10 is shown in FIG. 11. FIG.11 is different from FIG. 10 in that the nonlinear reactors 1a and d areinductively disposed on a common magnetic core (not shown) while thenonlinear reactors lb and c are inductively disposed on another commonmagnetic core (not shown). Further, a pair of serially connectednonlinear reactors la and c or lb and d are connected at both ends to apair of main diodes 4a and c or 4b and d. Then the diodes 4a and c areconnected at the cathode electrodes to the positive terminal P of thesource E while the diodes 4c and a are connected at the anode electrodesto the negative terminal N of the source.

This connection of the main diodes are particularly effective forpreventing the capacitors 3a and b from overcharging in the case thenonlinear reactors are high in residual inductance, that is, inferior insaturation characteristic.

More specifically, considering the process effected after thecommutation mode of operation in which the thyristors 2a and d have beenbrought into their nonconducting state, the main diodes 4a and d arefired when the energy accumulated in the saturated in inductance of eachreactor la or d tends to additionally charge the associated capacitor 3aor b with the polarity illustrated in FIG. 11. This causes the cathodeand anode electrodes respectively of the thyristors 2a and d to beclamped at the respective potentials at the source terminals P and Nwith the result that the increased voltages across the capacitors 3a andb remain substantially equal in magnitude to the voltage across thesource regardless of any voltage induced through the release of theenergy accumulated in the saturated inductance of each nonlinear reactortherefrom. Those released energies are delivered to the load (not shown)through the output terminal 0.

FIG. 12 illustrates another modification of the invention using acommutation trans former. As shown, a pair of serially connectedcommutation capacitors 3a and b are connected across and dc inputterminals P and N and the junction of both capacitors is connected to anintermediate tap on a commutation transformer shown as being anautotransformer 8. The transformer 8 include a pair of seriallyconnected windings 8a and b and both end terminals connected to thejunction of the nonlinear reactor 1a and the thyristor 2a seriallyconnected to each other and across the main semiconductor diode 4a andto the junction of the nonlinear reactor 1b and the thyristor 2bserially connected to each other and across the main semiconductor diode4b. The diodes 4a and b and therefore a pair of reactor and thyristorcombinations 1a, 2a and 1b, 2b are connected across the terminals P andN and the junction of the diodes 4a and b is connected to the load (notshown) through the output terminal 0.

It is now assumed that the thyristor 2a is in its conducting state tocause a load current i to flow from the positive source terminal Pthrough the now conducting thyristor 2a, the nonlinear reactor 1a andthe output terminal 0 to the load (not shown). The load may be connectedbetween the output tenninal O and any one of the terminals P and N andthe neutral point of the dc source (not shown as the case may be. Thisis true in the case of various examples including no output transformeror being of the bridge type. Under the assumed condition, the voltageacross the capacitor 3a is of a zero magnitude while the voltage acrossthe capacitor 3b has the magnitude of voltage across the terminals P andN. Further the nonlinear reactor la has been saturated due to the flowof load current i,, therethrough and the nonlinear reactor lb is put inits unsaturated flux region as in the previous examples.

Then upon firing the thyristor 2b, a voltage accumulated on thecommutation capacitor 3b is applied to the transformer winding 8b of thecommutation transformer 8 thereby to induce across the winding a voltageequal in magnitude and polarity to the applied voltage. As a result, thedoubled voltage is applied to the thyristor 2a to render itnonconductive. At that time both a closed loop including the components3b 8b 2b and a closed loop including the components P 3a 8b 2b N form aprimary winding with respect to the commutation transformer 8 while thecomponents 8a 1a O as well as the components 8a 1a lb forms a secondarycircuit with respect to the same transformer.

As the capacitors 3b and a discharge and charge respectively, thethyristor 2a decreases in cathode potential leading to the applicationof a forward voltage to the same. The capacitors 3a and b continue tocharge and discharge respectively until the voltage across the capacitor3b reaches zero magnitude. At that time the output temiinal is at avoltage equal to the potential at the source terminal N which initiatesfiring of the diode 4b.

On the other hand, the nonlinear reactor la has its saturated residualinductance on which a some energy has been eccumulated corresponding tothe abovementioned flow of load current therethrough. Therefore thatreactor has induced thereacross a voltage causing the cathode electrodeof the thyristor 2a to be negative with respect to the anode electrodethereof (as shown at the arrow beside the reactor 1a in FIG. 12). Theinduced voltage causes the magnetic flux in the reactor 1a to be resetfrom its saturated region to its unsaturated region with the result thatthe voltaged applied to the thyristor 2a is higher than the sourcevoltage by a magnitude corresponding to the voltage required forresetting the magnetic flux.

When that resetting voltage tends to decrease the potential at thecathode electrode of the thyristor 2a below the potential at the sourceterminal N (which maintains both the thyristor 2b and the diode 4bconducting), the commutation capacitor 3b is charged to one half theresetting voltage but with the polarity reversed from that illustratedwhereas the capacitor 3a is charged to a voltage higher than the sourcevoltage by one half the resetting voltage. In other words, the resettingof the magnetic flux in the nonlinear reactor la to its unsaturatedregion is accomplished in a parallel oscillation circuit formedequivalently of the nonlinear reactor 1a and the commutation capacitors3a and b through the commutation transformer 8.

Then when the load current is reversed in polarity. The nonlinearreactor 1b is now saturated and the next half cycle of the load currentflows through the output terminal 0, the reactor 1b and the thyristor 2bafter which the process as above described is repeated.

FIGS. 13 and 14 show different modifications of the system illustratedin FIG. 12. In FIG. 13, the autotransformer 8 is replaced by atransformer 8 including a primary and a secondary winding 8a and brespectively and the capacitors 3a and b are connected to both windings.At one end, in other respects the arrange ment is identical to thatshown in FIG. 12.

In FIG. 14, the thyristor 2a or b inter changes the associated nonlinearreactor 1a or b in position and the capacitors 3a and b are connected tothe transformer 8 at both ends rather than at the intermediate tap. Inother respects the arrangement is identical to that shown in FIG. 12.

FIG. 15 illustrates still another modification of the system shown inFIG. 12. Only a difference between both the systems is in that in FIG.15 a pair of clamping semiconductor diodes 9a and b of similar polarityare connected to a pair of serially connected nonlinear reactors 1b anda for the purpose of preventing the voltage across each thyristor 2a orb from exceeding the voltage across the terminals P and N. That is, theanode and cathode electrodes respectively of the clamping diode 9a and bare connected to the ends of i the serially connected reactors lb and abetween the source terminals P and N.

In the arrangement of FIG. 12 the nonlinear rector 1a or b cooperateswith the capacitors 3a and b to effect an oscillation on the latterportion of the commutation time. Therefore the voltage across theassociated thyristor can overshoot the voltage across the terminals Pand N, and the energy accumulated on the reactor la is transferred tothe capacitor. In the arrangement of FIG. 15, however, when the voltageacross the thyristor 2a or the capacitor. 3b is in excess of the sourcevoltage the diode 9a is fired. If an energy previously accumulated onthe saturated inductance of the reactor 1a is still left in the latterat that time, the energy is delivered to the load (not shown) throughthe output terminal 0.

After the nonlinear reactor 1a has terminated to release the accumulatedenergy therefrom the diode 4a becomes nonconducting and instead thediode 4b is initiated to be conducting. Under these circumstances theload current flows through both a current path including the diode 9a,the reactor la and the output terminal O and another current pathincluding the diode 4b and the output terminal 0. The former currentpath has a relatively high resistance so that the current portionflowing through that path is gradually transferred to the latter currentpath. Then the load current is decreased or reversed in polarity untilit passes through the zero current point. Then the load current beginsto flow through the thyristor 2a and the associated components.

respectively Thus it will be appreciated that the clamping diode 9aserves to prevent the potential at the cathode electrode of thethyristor 2a from dropping below the potential at the source terminal Nwhile the clamping diode 9b serves to prevent the potential at the anodeelectrode of the thyristor 2b from rising beyond the potential at thesource terminal P.

The waveforms of voltage and current developed in the arrangement ofFIG. 15 are shown in FIG. 16 wherein the reference characters have thesame meaning as in-FIG. l or 3. Voltage waveform A indicates that thevoltage applied across the thyristor is prevent from overshooting.

FIG. 17 is a view similar to FIG. 15 except for the diodes 4a and bbeing omitted. The arrangement shown in FIG. 17 has be found to beparticularly effective for use with loads lagging in power factor. Morespecifically, the load current serves to rapidly discharge the capacitor3b while the nonlinear reactor lb associated with the now conductingthyristor is prevented from being saturated. This prevents the currentflowing through the nonlinear reactor 1a from increasing above the loadcurrent.

Therefore a quantity of energy accumulated on the nonlinear reactor isnot only very small but also it is delivered to the load while itdecreases along with the load current. This is'because the othernonlinear reactor 1b is not saturated before the load current isreversed in flow direction. Therefore the energy accumulated on theindividual reactor is not lost.

This is true in the case the system of FIG. 12 includes the commutationtransformer.

FIG. 18 shows an arrangement similar to that shown in FIG. 17 exceptingthat one commutation diode 6a or b is connected in reverse parallelcircuit relationship to each of the thyristors 2a or b with thecommutation capacitors 3a and b connected to the intermediate point onthe commutation transformer 8 through a common commutation reactor 7 ofair core type.

When the thyristor 2a for example, is maintained reversely biased thecommutation diode 6a is put in its conducting state. The capacitors 3aand b cooperates with the commutation reactor 7 to effect oscillatorycharging and discharging respectively whereby the commutationtransformer winding 8a has a pulse current in the form of a sinusoid athalf-wave flowing toward the cathode electrode of the thyristor 2atherethrough with a pulse recurrence frequency determined by themagnitudes of the commutation capacitors and reactor. During a period oftime when the pulse current which might flow through the reactor la ishigher than that portion of the load current in the just preceding halfcycle, the commutation diode 6a is in its conducting state while thethyristor 2a is reversely biased resulting in the improvements incommutation characteristics.

It is noted that the diodes 9a and b serves, in addition to effectingthe clamping function as previous described, to perform the operation ofthe main diodes 4a and b, that is, the operation of feeding back thereactive power of the load. It is also noted that, with the diodes 9aand b omitted the commutation diodes la and b can also perform theoperation of the main diodes 4a and b respectively.

An arrangement shown in FIG. 19 is different from that illustrated inFIG. 18 only in that the communication diodes 6a and b each areconnected across the associated thyristor 2a or b through that windingportions having a small number of turns of the respective non linearreactor la or b. This measure permits the thyristor to be reverselybiased with a higher voltage equal to the sum of the forward voltagedrop across the diode and a voltage developed across the winding portionof the associated reactor as above described. As a result, thearrangement is advantageous in that the movement of the carriers in thesemiconducti-ve material for the thyristor is accelerated upon renderingthe thyristor nonconducting thereby to additionally reduce the tum-offtime thereof.

In order to prevent the voltage applied across the thyristor fromexcessively increasing due to the resetting voltage developed across thenonlinear reactor and. also to accelerate resetting of the magnetic fluxin the latter, any of arrangements shown in FIGS. 20 through 23 may beeffectively used.

The arrangement of FIG. 20 is substantially similar to that shown inFIG. 12 except for a series combination of a semiconductor diode 10 anda damping resistor 11 being connected across a pair of nonlinearreactors 1a and b serially connected to each other.

As previously descirbed in conjunction with FIG. 12, the resettingvoltage developed across the nonlinear reactor la upon its resettingtends to cause the voltage applied to the thyristor 2a to be higher thanthe source voltage and also to decrease the potential at the cathode ofthat thyristor below the potential at the source terminal N. Thereby thecommutation capacitor 3b is charged with the polarity reverse from thatillustrated leading to the development of a reverse voltage across thecommutation transformer 8. At that time the diode 10 is tired to connectthe resistor 11 across the serially coneected reactors 1a and bresulting in the suppression of the resetting'voltage. This means thatthe nonlinear reactors are reset to its unsalurated flux region innon-oscillatory manner rather than in oscillatory manner as in FIG. 12.Therefore the arrangement rapidly reaches its steady state operation ascompared with that shown in FIG. 12.

As previously described at least one of the serially connected nonlinearreactors la and b having the series combination of diode and resistor 10and 11 respectively connected thereacross, for example, the reactor lbis put in its unsaturated flux region to exhibit a high impedance. Thatis it is maintained excited with a low current. Therefore during theresetting of both reactors a current flowing through the diode l and theresistor 11 is approximately equal in magnitude to the low excitingcurrent as above described. This permits the magnitude of resistance 11to be selected at will within limits of the desired rate at which themagnetic fluxiis reset and of the desired extent to which the voltageapplied to each across Rach thyristor can increase.

The arrangement of FIG. 21 is substantially similar to that shown inFIG. 8 in which the series combination of diode and damping resitor 10and 11 is connected in parallel circuit relationship with the seriallyconnected nonlinear reactors 1a and b. Therefore the diode and resistorcombination is equivalently coupled across one half the total windingsof both the reactors, assuming that the primary winding is equal innumber of turns to the secondary winding for each reactor. Therefore ascompared with the arrangement of FIG. 20, the damping resitor I1 ispreferably selected to have a resistance decreased by a factor of fourso that the current flowing therethrough is doubled while the resultingvoltage drop thereacross is halved. In other respects the arrangement isidentical to that shown in FIG. 20. If desired, the seried diodes 5a, b,c and d may be omitted.

FIG. 22 shows another modification of the system illustrated in FIG. 2or 20. The nonlinear reactors la and b are provided with secondarywindings la, and 1b, serially interconnected through the seriescombination of diode and damping resistor 10 and 11. In other respectsthe arrangement is substantially similar to that shown in FIG. 2, andtherefore its operation will be readily understood from the descriptionas previously made for FIGS. 2 and 20.

FIG. 23 shows another modification of the system illustrated in FIG. 12.The commutation autotransformer 8 connected across a pair of seriallyconnected nonlinear reactors 1a and b is provided with a secondarywinding 8s connected across the terminals P and N through asemiconductor diode 10. In other respects the arrangement is identicalto that shown in FIG. 12.

In the arrangement of FIG. 23, whatever low power results from theresetting of the magnetic flux is fed back to minimize a loss of energywhile the resetting of the magnetic flux is accomplished at apredetermined constant rate and hence more rapidly.

If desired, the nonlinear reactors la and b may be provided withrespective secondary windings connected to the terminals P and N throughthe diode 10 with the secondary commutation winding 8s omitted.Alternatively the secondary windings just described may be seriallyconnected together and across the terminals P and N through the diode10.

The means for suppressing a rate of change in magnetic flux as abovedescribed is effective for rapidly resetting the magnetic flux withoutany increase in thyristor voltage or power loss. Thus the operation ispossible to be performed at higher frequencies. For example, as comparedwith the systems including no means for suppressing a rate of change inmagnetic flux, the operating frequency has increased by a factor of twoor three for FIGS. 20 through 22 and by a factor of about four to fivefor FIGS. 23. Although a power loss has been scarcely different betweenthe presence and absence of such means at the order of commercialfrequency a disparity therebetween has been a several times as large ata frequency of from 200 to 500 Hz at which the flux resetting somes intoquestion.

In order for the systems as above described to be simple in constructionin expensive to be manufactured while improving the capability, thenonlinear reactors and commutation transformer can be formed into aunitary structure such as shown in FIG. 24 wherein the system of FIG. 12is also illustrated. A shell type magnetic core generally designated bythe reference numeral 12 has a pair of outer legs 12a and b on which apair of nonlinear reactor windings 1a and b are inductively disposed anda central leg 12c on which a pair of commutation .windings 8a and b areinductively disposed in series circuit relationship.

If desired, the central core leg 120 may have inductively disposedthereon the secondary windings 1a, and lb, for suppressing a resettingrate for magnetic flux as shown in FIG. 22 or the secondary windings 1a,and lb, as shown in FIG. 23. Further the central core 120 may haveinductively disposed thereon a dc biasing winding as will be describedhereinafter.

FIG. 25 shows another transformer including a pair of superposedmagnetic cores 12a and b in the form of toroids, the nonlinear windings1a and b inductively disposed on the toroidal cores 12a and brespectively, the commutation windings 8a and b inductively disposedaround the nonlinear windings la and b, and a dc biasing winding 13inductively disposed around the windings 8a and b.

The toroidal core is generally formed of any suitable magnetic materialhigh in magnetic properties. Its use leads to a decrease in commutationloss, and controls operative at high frequencies. Due to the core beingcontinuous, the noise is much decreased. Also such core can have theperfectly rectangular hysteresis loop thereby to decrease the residualinductance of the associated nonlinear reactor and thereforeoscillations duce to that residual inductance. With the rectangularhysteresis loop the biasing winding serves to reset the magnetic flux inthe core.

In summary, at least two thyristor serially connected to the nonlinearreactors respectively are alternately turned on and off so that avoltage charged on the commutation capacitor is reversely applied acrossthe previously conducting thyristor through the now conducting thyristorwhile during that reversely biasing time that nonlinear reactor seriallyconnected to the now conducting thyristor is in its unsaturated fluxregion thereby to maintain a high impedance between the output terminaland the dc source terminal orbetween the source terminals.

Under these circumstances, it is required to reset the magnetic flux ineither of the nonlinear reactor during a period of time for which thethyristor serially connected to that reactor is in its nonconductingstate. An extent to which the magnetic flux is reset depends upon thefunction of the nonlinear reactor for holding the succeedingunsaturation and corresponds to a change in magnetic flux capable ofbeing utilized when the unsaturation is held.

If separate magnetic cores are used to form different nonlinear reactorsoperated in the manner just described the operation of each magneticcore is expressed by solid hysteresis loop shown in FIG. 26 wherein theaxis of ordinates represents the magnetic induction B or flux densityand the axis of abscissas represents the magnetic field intensity H.From FIG. 26 it is seen that the resetting is accemplished to theremanence Br inherent to the magnetic material in volved and that achange Ad) in magnetic flux avaiable equals a difference between thesaturated flux density Bs and the remanence Br. Therefore the effectiveresetting range has been rather narrow.

Further a rate at which the magnetic flux increases in the nonlinearreactor is extremely high corresponding to the tum-off time of thethyristor and equivalent to a high frequency. Therefore it is desirableto render the high frequency characteristics good while decreasing themagnetomotive force because of a small number of turns of the associatedwinding. This desirability generally has resulted in increases inremanence Br and flux density difference Adv which is, in turn,accompanied by the large-sized nonlinear reactor.

In order to eliminate those disadvantages, the nonlinear reactors can beeffectively formed into a unitary structure as diagrarnatically shown inFIG. 27 or 28. FIG. 27 shows a single shell type magnetic core 13'including a pair of outer legs on which a pair of nonlinear reactorwindings la and b are inductively disposed. In the arrangementillustrated magnetic paths established in the core with currents flowingthrough the nonlinear reactors la and b during different periods of timeare partly common to each other.

It is now assumed that a current is flowing through the nonlinearreactor 1a to generate a magnetic flux of P in the core as shown atsolid line in FIG. 27 until the core reaches magnetic saturation. Atthat time a shunt flux 1 as shown at dot line in FIG. 27 is caused toflow toward the nonlinear reactor lb to reset it. In this event the corematerial has a hysteresis loop such as shown at solid line in FIG. 29.When the current flowing through the reactor lb passes through the zerocurrent point the resetting is effected from the positive saturation Bsto the remanence Br and further from the remanence Br to a point x inthe negatively unsaturated region in the process of flowing the loadcurrent through the nonlinear reactor la (see FIG. 29). Even if the saidload current decreases before the succeeding commutation the magneticflux density remain at a point y resulting in the effective change influx increasing as shown at A4) in FIG. 29.

In FIG. 28, a pair of core type magnetic cores 14 and 15 juxtaposed toeach other substitutes the single shell type core 13' as shown in FIG.27. The greater parts of the nonlinear reactor 1a, b windings are woundaround the outer legs of the juxtaposed cores while the winding on eachcore is partly wound around the other leg of the other core. In thearrangement illustrated, the load current of large magnitude flowsthrough one of the reactors and also through that portion thereof smallin number of turns and disposed on the other core thereby to reset thatcore.

The arrangement is advantageous in that as the resetting can be effectedin the vicinity of the negative saturation-Es the negative remanence-Brcan be utilized in the succeeding commutation. Therefore if a magneticmaterial having a rectangular hysteresis loop is used, that-Br isapproximately equal to the negative sturation-Bs to permit theutilization of the substantially entire change in magnetic flux.

In addition the dc biasing winding as above described may be equallyused to expand the range within which the magnetic flux can change.

While the invention has been illustrated and described in conjunctionwith various preferred embodiments thereof it is to be understood thatnumerous changes and modifications may be resorted to without departingfrom the sperit and scope of the invention. For example, it is to benoted that the invention is not restricted to the use of the nonlinearreactors and that any suitable linear reactors may be used withsatisfactory results excepting that a power loss due to the energyaccumated on the reactor is somewhat increased Further the invention hasbeen illustrated and described in terms of direct current-to-alternatingcurrent inverters in which the source terminals P and N serve to supplya dc voltage and an ac output is developed at the output terminal or theoutput terminals O and 0 However it is to be understood that theinvention is not restricted to such inverters and that it is equallyapplicable to the so-called DC-to-DC converters or choppers wherein aload is connected between the output terminal 0 and the source terminalP or N or between the output terminals 0 and O and wherein at least twothyristors change in conduction ratio therebetween to control the dcpower supplied to the load. In order to control the conduction ratio,the thyristors may be alternately turned on and off at a high frequencyand the conduction ratio is modulated with a frequency sufficiently lessthan the on-off frequency. This measure is known as the high frequencypulse width modulation DC-AC inverters.

What is claimed is:

1. A power control system comprising, in combination, a pair of directcurrent (dc) source terminals, a pair of alternating current (ac) outputterminals, first and second series circuits each connected at one end toone of said dc terminals and coupled at their other ends, respectively,to said ac terminals, wherein each said series circuit comprises asemiconductor controlled rectifier and a non-linear reactor, meansconnecting the other of said do terminals in a current carrying pathwith said first and second series circuits, and a pair of semiconductordiodes connected, respectively, in parallel with said controlledrectifiers and in a reverse polarity relationship therewith, a thirdseries circuit comprising a commutation reactor and a commutationcapacitor connected at one end to the junction of one of said nonlinearreactors and controlled rectifiers, so that said controlled rectifiersare switched sequentially to provide an ac output voltage at said acterminals.

2. A power control system as claimed in claim 1, wherein the other endof said series combination of said commutation capacitor and reactor. isconnected to the junction of the other of said semiconductor controlledrectifiers and its associated nonlinear reactor.

3. A power control system as claimed in claim 1, further comprisingthird and fourth semiconductor diodes connected respectively in parallelwith said first and second series circuits and in opposed polarity withthe controlled rectifiers in said first and second series circuits.

4. A power control system as claimed in claim 1, wherein said nonlinearreactors are provided with respective secondary windings and a fourthseries combination including a fifth semiconductor diode and a dampingresistor connected between said secondary windings.

5. A power control system comprising, in combina tion, a pair of directcurrent (dc) source terminals, a pair of alternating current (ac) outputterminals, first and second series circuits each connected at one end,respectively, to said dc terminals and coupled at their other ends toone of said ac terminals, wherein each said series circuit comprises asemiconductor controlled rectifier and a nonlinear reactor,

and a pair of semiconductor diodes connected respectively, in parallelwith said controlled rectifiers and in a reverse polarity relationshiptherewith, a commutation circuit including a pair of commutationcapacitors, a commutation transformer having first and secondmagnetically coupled windings, and circuit means for connecting saidfirst and second capacitors and windings to said controlled rectifiersfor supplying an ac current path to said ac terminals due to alternateswitching of said controlled rectifiers.

6. A power control system as claimed in claim 5, wherein said twocommutation capacitors are serially connected across said pair of directcurrent source terminals, and one of said commutation transformerwindings is a tapped winding connected across the junctions of saidserially connected semiconductor controlled rectifiers and nonlinearreactors, the junction of said commutation capacitors being connected tothe tap on said one transformer winding.

7. A power control system as claimed in claim 5, wherein said twocommutation capacitors are serially connected across said pair of directcurrent source terminals, and one of said commutation transformerwindings is a tapped winding connected across the junctions of saidserially connected semiconductor controlled rectifiers and nonlinearreactors, the junction of said commutation capacitors being connected tothe tap on said one transformer winding, and wherein a semiconductordiode and a control resistor are connected in series across thejunctions of said serially connected semiconductor controlled rectifiersand nonlinear reactors.

8. A power control system as claimed in claim 5, wherein saidcommutation transformer further includes a third winding and a thirdsemiconductor diode connected in series across said pair of directcurrent source terminals.

9. A power control system as claimed in claim 5, further comprisingmagnetic coupling means for interlinking said first and secondtransformer windings and said first and second reactors.

10. A power control system as claimed in claim 5, further comprising athree-leg magnetic core having the windings of said nonlinear reactorsinductively disposed respectively on the outer legs thereof, and saidfirst and second windings of said commutation transformer inductivelydisposed on the central leg thereof.

11. A power control system as claimed in claim 5, further comprising atleast two toroidal magnetic cores which are axially superposed and havethe windings of said nonlinear reactors inductively disposedrespectively thereon, said first and second windings of said commutationtransformer being inductively disposed on both said magnetic cores.

12. A power control system as claimed in claim 1, further comprising athree-leg magnetic core having the windings of said first and secondreactors inductively disposed, respectively, on the outer legs of saidmagnetic core.

13. A power control system as claimed in claim 1, further comprising apair of magnetic cores, wherein each of said nonlinear reactors includesa first winding inductively disposed on a different one of said magnetic

1. A power control system comprising, in combination, a pair of directcurrent (dc) source terminals, a pair of alternating current (ac) outputterminals, first and second series circuits each connected at one end toone of said dc terminals and coupled at their other ends, respectively,to said ac terminals, wherein each said series circuit comprises asemiconductor controlled rectifier and a non-linear reactor, meansconnecting the other of said dc terminals in a current carrying pathwith said first and second series circuits, and a pair of semiconductordiodes connected, respectively, in parallel with said controlledrectifiers and in a reverse polarity relationship therewith, a thirdseries circuit comprising a commutation reactor and a commutationcapacitor connected at one end to the junction of one of said nonlinearreactors and controlled rectifiers, so that said controlled rectifiersare switched sequentially to provide an ac output voltage at said acterminals.
 2. A power control system as claimed in claim 1, wherein theother end of said series combination of said commutation capacitor andreactor is connected to the junction of the other of said semiconductorcontrolled rectifiers and its associated nonlinear reactor.
 3. A powercontrol system as claimed in claim 1, further comprising third andfourth semiconductor diodes connected respectively in parallel with saidfirst and second series circuits and in opposed polarity with thecontrolled rectifiers in said first and second series circuits.
 4. Apower control system as claimed in claim 1, wherein said nonlinearreactors are provided with respective secondary windings and a fourthseries combination including a fifth semiconductor diode and a dampingresistor connected between said secondary windings.
 5. A power controlsystem comprising, in combination, a pair of direct current (dc) sourceterminals, a pair of alternating current (ac) output terminals, firstand second series circuits each connected at one end, respectively, tosaid dc terminals and coupled at their other ends to one of said acterminals, wherein each said series circuit comprises a semiconductorcontrolled rectifier and a nonlinear reactor, and a pair ofsemiconductor diodes connected respectively, in parallel with saidcontrolled rectifiers and in a reverse polarity relationship therewith,a commutation circuit including a pair of commutation capacitors, acommutation transformer having first and second magnetically coupledwindings, and circuit means for connecting said first and secondcapacitoRs and windings to said controlled rectifiers for supplying anac current path to said ac terminals due to alternate switching of saidcontrolled rectifiers.
 6. A power control system as claimed in claim 5,wherein said two commutation capacitors are serially connected acrosssaid pair of direct current source terminals, and one of saidcommutation transformer windings is a tapped winding connected acrossthe junctions of said serially connected semiconductor controlledrectifiers and nonlinear reactors, the junction of said commutationcapacitors being connected to the tap on said one transformer winding.7. A power control system as claimed in claim 5, wherein said twocommutation capacitors are serially connected across said pair of directcurrent source terminals, and one of said commutation transformerwindings is a tapped winding connected across the junctions of saidserially connected semiconductor controlled rectifiers and nonlinearreactors, the junction of said commutation capacitors being connected tothe tap on said one transformer winding, and wherein a semiconductordiode and a control resistor are connected in series across thejunctions of said serially connected semiconductor controlled rectifiersand nonlinear reactors.
 8. A power control system as claimed in claim 5,wherein said commutation transformer further includes a third windingand a third semiconductor diode connected in series across said pair ofdirect current source terminals.
 9. A power control system as claimed inclaim 5, further comprising magnetic coupling means for interlinkingsaid first and second transformer windings and said first and secondreactors.
 10. A power control system as claimed in claim 5, furthercomprising a three-leg magnetic core having the windings of saidnonlinear reactors inductively disposed respectively on the outer legsthereof, and said first and second windings of said commutationtransformer inductively disposed on the central leg thereof.
 11. A powercontrol system as claimed in claim 5, further comprising at least twotoroidal magnetic cores which are axially superposed and have thewindings of said nonlinear reactors inductively disposed respectivelythereon, said first and second windings of said commutation transformerbeing inductively disposed on both said magnetic cores.
 12. A powercontrol system as claimed in claim 1, further comprising a three-legmagnetic core having the windings of said first and second reactorsinductively disposed, respectively, on the outer legs of said magneticcore.
 13. A power control system as claimed in claim 1, furthercomprising a pair of magnetic cores, wherein each of said nonlinearreactors includes a first winding inductively disposed on a differentone of said magnetic cores and a second winding disposed on the opposedmagnetic core and having flowing therethrough a load current passingthrough the other nonlinear reactor for resetting the magnetic fluxes insaid nonlinear reactors.